Wavelength-division multiplexing (WDM) systems place several different wavelengths of laser radiation into a single optical fiber (multiplexing) at the transmitter, and spatially separate the wavelengths at the receiver (demultiplexing). Most previous wavelength-division multiplexing schemes are either expensive and bulky, or suffer from poor insertion loss characteristics.
One previous approach is to use a "star coupler", in which all of N (an integer number of) wavelengths are sent to N different optical fibers. In each optical fiber, an in-line Fabry-Perot filter passes one wavelength only. This approach, in addition to being expensive because of the fiber-Fabry-Perot filters, has a very poor insertion loss of 1/N, where N is the number of wavelengths (i.e., channels). For N channels, an insertion loss of 1/N means that each channel has been attenuated by a factor of 1/N after the combining process.
Other multiplexer/demultiplexer schemes include a monolithic array grating ("MAGIC") demultiplexer which is described by Soole et al., in an article entitled "Wavelength Precision of Monolithic InP Grating Multiplexer/Demultiplexers", Electronics Letters, Apr. 14, 1994, Vol. 30, No. 8, pp. 664-665. A waveguide routing approach is described by Bissessur et al., in an article entitled "Tunable Phased-Array Wavelength Demultiplexer on InP", Electronics Letters, Jan. 5, 1995, Vol. 31, No. 1, pp. 32-33. The MAGIC device is essentially a curved on-chip grating, which simultaneously focuses the incoming light and reflects beams to separate locations depending on wavelength. The wavelength router allows multiple wavelengths to diffract and overlap, with constructive interference occurring in separate spatial locations depending on the wavelength. Both the MAGIC and waveguide routing approaches require several centimeters of on-wafer length and have high insertion loss characteristics.